[augeas-devel] Distinguishing empty string from no value in path expressions

[David Lutterkort]

On Thu, 2011-10-06 at 21:25 +1100, Michael Chapman wrote:
> On Wed, 5 Oct 2011, David Lutterkort wrote:
> > So far, nobody has needed that distinction, but it would be easy enough
> > to add a null function to the language so that you could pick out nodes
> > with null values, i.e. something like
> >
> >   match test/*[null(.)]
> >
> > which, in your example, would match /test/b
> 
> A null() function would be useful, however I'm not sure it should take a 
> nodeset as an argument. It doesn't make sense for it to be applied to more 
> than one node.

The reason I said nodeset is that there's no type internally for just a
node. There's two ways we can deal with passing more than one node: (1)
return true only if all the nodes have null values (2) throw an error.

I am inclined to do (1)

> Perhaps it should be a no-argument predicate, "this node has no value".

It would be more flexible if it takes an argument; I might do both, i.e.
make the argument to null() optional.

> Another question arises from all this: how would you match only non-null 
> nodes?

Good point. We seem to be missing the boolean 'not' operator. I am very
tempted to do what XPath does and make not a function. With that, you'd
write

    match /test/*[not(null())]

David